rt_dptbl(4) UNIX System V rt_dptbl(4)
NAME
rtdptbl - real-time dispatcher parameter table
DESCRIPTION
The process scheduler (or dispatcher) is the portion of the kernel that
controls allocation of the CPU to processes. The scheduler supports the
notion of scheduling classes where each class defines a scheduling
policy, used to schedule processes within that class. Associated with
each scheduling class is a set of priority queues on which ready to run
processes are linked. These priority queues are mapped by the system
configuration into a set of global scheduling priorities which are
available to processes within the class. (The dispatcher always selects
for execution the process with the highest global scheduling priority in
the system.) The priority queues associated with a given class are
viewed by that class as a contiguous set of priority levels numbered from
0 (lowest priority) to n (highest priority-a configuration dependent
value). The set of global scheduling priorities that the queues for a
given class are mapped into might not start at zero and might not be
contiguous (depending on the configuration).
The real-time class maintains an in-core table, with an entry for each
priority level, giving the properties of that level. This table is
called the real-time dispatcher parameter table (rtdptbl). The rtdptbl
consists of an array of parameter structures (struct rtdpent), one for
each of the n priority levels. The properties of a given priority level
i are specified by the ith parameter structure in this array (rtdptbli).
A parameter structure consists of the following members. These are also
described in the /usr/include/sys/rt.h header file.
rtglobpri
The global scheduling priority associated with this priority
level. The mapping between real-time priority levels and global
scheduling priorities is determined at boot time by the system
configuration. The rtglobpri values cannot be changed with
dispadmin(1M).
rtquantum
The length of the time quantum allocated to processes at this
level in ticks (HZ). The time quantum value is only a default or
starting value for processes at a particular level as the time
quantum of a real-time process can be changed by the user with
the priocntl command or the priocntl system call.
An administrator can affect the behavior of the real-time portion of the
scheduler by reconfiguring the rtdptbl. There are two methods available
for doing this.
DISPADMIN CONFIGURATION FILE
The rtquantum values in the rtdptbl can be examined and modified on a
running system using the dispadmin(1M) command. Invoking dispadmin for
the real-time class allows the administrator to retrieve the current
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rt_dptbl(4) UNIX System V rt_dptbl(4)
rtdptbl configuration from the kernel's in-core table, or overwrite the
in-core table with values from a configuration file. The configuration
file used for input to dispadmin must conform to the specific format
described below.
Blank lines are ignored and any part of a line to the right of a # symbol
is treated as a comment. The first non-blank, non-comment line must
indicate the resolution to be used for interpreting the time quantum
values. The resolution is specified as
RES=res
where res is a positive integer between 1 and 1,000,000,000 inclusive and
the resolution used is the reciprocal of res in seconds. (For example,
RES=1000 specifies millisecond resolution.) Although very fine
(nanosecond) resolution may be specified, the time quantum lengths are
rounded up to the next integral multiple of the system clock's
resolution. For example, the finest resolution currently available on
the 3B2 is 10 milliseconds (1 ``tick''). If res were 1000 a time quantum
value of 34 would specify a quantum of 34 milliseconds, which would be
rounded up to 4 ticks (40 milliseconds) on the 3B2.
The remaining lines in the file are used to specify the rtquantum values
for each of the real-time priority levels. The first line specifies the
quantum for real-time level 0, the second line specifies the quantum for
real-time level 1, etc. There must be exactly one line for each
configured real-time priority level. Each rtquantum entry must be
either a positive integer specifying the desired time quantum (in the
resolution given by res), or the symbol RTTQINF indicating an infinite
time quantum for that level.
EXAMPLE
The following excerpt from a dispadmin configuration file illustrates the
format. Note that for each line specifying a time quantum there is a
comment indicating the corresponding priority level. These level numbers
indicate priority within the real-time class, and the mapping between
these real-time priorities and the corresponding global scheduling
priorities is determined by the configuration specified in the rt master
file. The level numbers are strictly for the convenience of the
administrator reading the file and, as with any comment, they are ignored
by dispadmin on input. dispadmin assumes that the lines in the file are
ordered by consecutive, increasing priority level (from 0 to the maximum
configured real-time priority). The level numbers in the comments should
normally agree with this ordering; if for some reason they don't,
however, dispadmin is unaffected.
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rt_dptbl(4) UNIX System V rt_dptbl(4)
# Real-Time Dispatcher Configuration File
RES=1000
# TIME QUANTUM PRIORITY
# (rtquantum) LEVEL
100 # 0
100 # 1
100 # 2
100 # 3
100 # 4
100 # 5
90 # 6
90 # 7
. . .
. . .
. . .
10 # 58
10 # 59
FILES
/usr/include/sys/rt.h
SEE ALSO
dispadmin(1M), priocntl(1), priocntl(2),
``Scheduler'' chapter in the System Administrator's Guide
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